Hi there, I built this project (picutres to come) on a vero PC board with a 15 pico farad capacitor, and the NTSC colourburst crystal (3.795 etc Mhz)

I am not getting any colour from either the Composite out, or the Svideo jack. I'm guessing, from what I have read on the "Amiga RGB to S-Video + composite adaptor" thread that the capacitor might be the issue.

If the issue was just the svideo jack then the problem might be the chroma out put, ( bad connection etc ) but seeing as it's on both the composite (generated in chip) and the svideo, I'm guessing it's a timing issue with the crystal and capacitor. I will try a 20 pf.

If I need a variable one. what would be the best value to purchase?...

Last edited by waltermixxx; 05 January 2011 at 18:10.
Reason: change title a bit

he he, I was going to... one of the posts mentioned no cap at all
i was going to start with that... i have a bunch of small ones to try as well,
but a variable would allow me to fine tune incase it changes with another monitor...
but I suspect a a 0 to 20 pf would be good

Well now I am depressed... I could not find a cap that would allow it to sync up to this little LCD monitor... the picture remained in black and white, but I had issues with that monitor before... so I hooked it up to my 32" LCD HD tv, via svideo, and the blue signal came through fine, but the red and green looked odd. I tried various caps with that tv and either got black and white or a picture with messed up colours... I did verify my red blue green where going the appropriate pin n the chip, hsync and vsync appeared to be fine as I was getting a picture...

by the way I was testing this with my Atari 1040STE, I will have to fire up the Amiga 500 and see what kind of luck I get with that...

I noticed you have adapted the design with the addition of an LED, now the Amiga +5 on the Video out is 100mA (at maximum), the AD724JR requires a minimum of 42mA - its quite likely you are not providing enough power to run both devices.

Disconnect your 3.57 MHz crystal and use the NTSC carrier clock from pin 15 of the RGB port. This will be time aligned with the sync pulse edges, important for good colour definition (if possible with NTSC).

Have you ensured that pin 1 of the AD724 is pulled high to select NTSC encoding?
Is pin 12 pulled low to select FSC (3.57 MHz) clock input?

Normally a black and white picture is a result of an incorrect carrier frequency.

by the way I was testing this with my Atari 1040STE, I will have to fire up the Amiga 500 and see what kind of luck I get with that...

I have seen few people, and myself, having trouble getting STE to sync up to RGB monitors like 1084 and RGB composite converters. The sync in Atari is not as good as Amigas. I would try on Amiga first.
The black and white problem, looks like is related to color carrier between PAL and NTSC as stated above by others.

I am using it with an atari STE, mono detect line is floating so ST produces colour out.

I do have an amiga 500 I will try with as well, just to make sure it's not the circuit, as someone suggested that would be a good idea. I will post my findings... the Amiga does make it easy by providing a clock for the circuit... ( If it works with the amiga, I will remove the crystal and use it with my amiga, and perhaps doe the svideo mod on the ste.

I do have an atari colour rgb monitor, I was just hoping to use an LCD monitor as it's lighter and uses less electricity...

I was thinking of doing the Svideo mod to my 1040ste as it does have a modulator,
or simply using the composite video out on pin two of the 13 pin din, but the crisp image provided by svideo a million times better I'm just afraid of killing my STE doing the mod...

the 5 volts is coming from a power adapter, a switching variety for use with a USB hub, it's rated at 1 amp so plenty of power there for the LED i checked the voltage with a meter while powering the circuit and it's still 5 volts.

I may get 5 volts from the 15 pin ste joystick port... that may help too...

I will post how it goes with the Amiga 500. Thanks again...

The two pin socket is where i tried various caps, 10, 12, 15, 18, 22, 30 pf.
I'm sure there is a magic number in there somewhere

Last edited by TCD; 10 January 2011 at 09:04.
Reason: Back to back posts merged. Use the edit function.

Sorry for grave digging but I have just built this circuit and I searched all over the internet for an answer to this and eventually figured it out so I thought I'd give an answer.

I don't know what capacitor you have in your circuit but in the datasheet it will give you a 'shunt capcitance' to be used with it, typically 18 or 22pF.

Now, the crystal needs this much capacitance in parallel, but you need a capacitor value such that the cap value + any latent capacitance in your circuit = the shunt capacitance.

On PCBs the latent capacitance is quite small, but with strip/vero board the linear tracks cause _much higher_ latent capacitance in your circuit, much too high for the crystal, so in fact you probably already have far too much there.

I spent days playing with this and was going a bit crazy. I then realised this, and cut all the traces in my strip board connected to pin 3/the capcitor to make them as short as possible (no left over strip that goes nowhere). Before everything was black and white and blurry whatever I did, now its in perfect colour! Also, you can use a little trim capacitor to tune it to the right value, or just try some different values. Now I cut the traces I even get colour with no capacitor present.

Probably you've forgotten this project a long time ago, but maybe this can help someone out!